Thank you for your help Konstantina.
My *.Controls & *.Controls.Navigation assembly versions are both 2010.1.309.1030.
I checked and the icon is in the assembly (reflector). No problem there.
Prompt and Confirm both have the same issue.
I created a sample project but cannot reproduce my error. It displays correctly every time.
This is obviously something I am doing incorrectly in my project. The fact that adding your style in the xaml has no bearing on the RadWindow.Alert makes me think I have some overriding theme but I'm not familiar enough with XAML to figure out where that might be. I've searched the entire project for "theme" and found nothing.
If you have any troubleshooting ideas, I'd love to hear them, but if not, no worries.
I'm going to use the System.Windows.Browser.HtmlPage.Window.Alert for now,